When logging an earth formation, there are applications that require accurately measuring very small electromagnetic signals and phenomena. These measurements also require sensitive equipment. One such apparatus (tool) which has had practical success is a logging device which measures nuclear magnetic resonance ("NMR") properties of earth formations. Measurements of NMR characteristics of rock samples can be made in a laboratory with reasonable accuracy, but making comparable measurements in an earth borehole is rendered more difficult by the hostile borehole environments where temperatures may reach several hundred degrees Fahrenheit, pressures may reach thousands of p.s.i. and all of the equipment must be packed within a cylindrical volume of only several inches in diameter. A fundamental description of a well logging tool for measuring NMR characteristics is set forth in U.S. Pat. No. 4,933,638, assigned to the same assignee as the present application. Reference can also be made to U.S. Pat. Nos. 4,933,638 and 5,153,514 for descriptions of the conventional NMR logging approaches.
The NMR tool produces a static and substantially homogeneous magnetic field that is directed into an earth formation. By configuring and then directing the combined magnetic fields of a configuration of magnets in the NMR tool into the formation, a spatial field gradient substantially vanishes, thereby ensuring that the magnetic field is highly homogeneous throughout that region of the formation. The homogeneous magnetic field is several times stronger than the earth's magnetic field, which is thus imposed or focused on a volume of the formation in situ.
As stated, during this logging procedure, a static magnetic field is imposed on the formation. It has been recognized that particles of an earth formation such as atomic nuclei, protons or electrons have magnetic spins that tend to align with a static magnetic field B.sub.0. In NMR operations, if a pulse of alternating current having a frequency f is passed through a transmitter coil, thereby producing an oscillating polarizing field B.sub.1 perpendicular to the static field B.sub.0, a population of protons in the formation would be tipped away from the static field direction. At the end of the pulse, when B.sub.1 is removed, the protons precess about the B.sub.0 vector. After a characteristic time called the longitudinal or spin-lattice relaxation time T.sub.1, the protons will have relaxed to thermal equilibrium, wherein a percentage of protons are aligned in the direction of B.sub.0. Formation characteristics are measured and determined from the precess of the formation particles.
The basic NMR logging apparatus used in the context of the present invention is described in U.S. Pat. No. 5,153,514 which is incorporated by reference herein and is summarized in conjunction with FIGS. 1-3. In FIG. 1, a borehole 10 is shown adjacent to formations 11 the characteristics of which are to be determined. Within borehole 10 is a logging tool 13 in accordance with the referenced U.S. Pat. No. 5,153,514 which is connected via a wireline 8 to surface equipment 7. Tool 13 has a face 14 shaped to intimately contact the borehole wall, with minimal gaps or stand-off, and a retractable arm 15 which can be activated to press the body of the tool 13 against the wall's surface. A mudcake 16 is shown on the borehole wall. Although the tool 13 is shown as a single body, the tool may alternately comprise separate components such as a cartridge, sonde or skid, and the tool may be combinable with other logging tools. Also, while a wireline is illustrated, alternative forms of physical support and communicating link can be used, for example in a measurement while drilling system. The tool 13 includes a magnet array 17 and an antenna 18 positioned between the array 17 and the wall engaging face 14. Magnet array 17 produces a static magnetic field B.sub.0 in regions surrounding the tool 13. The antenna 18 produces, at selected times, an oscillating magnetic field B.sub.1 which is focused into formation 12, and is superposed on the static field B.sub.0 within those parts of the formation opposite the face 14. The "volume of investigation" of the tool shown in dotted lines in FIG. 1, is a vertically elongated region directly in front of tool face 14 in which the magnetic field produced by the magnet array 17 is substantially homogeneous and the spatial gradient thereof is approximately zero. The tool 13 makes a measurement by magnetically tipping the nuclear spins of particles in formation 12 with a pulse of oscillating field B.sub.1, and then detecting the precession of the tipped particles in the static, homogeneous field B.sub.0 within the volume of investigation over a period of time.
FIG. 2 shows a magnet array 17 disclosed in the apparatus of the above-referenced patent. The magnet array includes three permanent magnets 24, 25 and 26 which are mounted parallel to each other within a metal alloy body 27. The body 27 is a material having low magnetic permeability, so as to not interfere with the static magnetic field. Magnets 24, 25 and 26 are elongated in the longitudinal direction of the borehole. The magnetic poles of each magnet are not on the smallest faces of the slab, commonly viewed as the ends of a bar magnet; instead, the poles appear on the two opposing edges of the slab magnet and point to the left and right respectively. Therefore, within the formation 12, the magnetic field B.sub.0 surrounding the magnets remains fairly constant along the longitudinal direction of the borehole axis. In the illustration of FIG. 2, magnets 24 and 26 are symmetrically mounted in the two sides of the body 27 with the north poles facing the same directions. Magnet 25 is positioned parallel to and between the other two magnets, but with its north poles facing oppositely from magnets 24 and 26. Magnet 25 is also shifted slightly away from face 14, relative to magnets 24 and 26. The north poles of magnets 24 and 26 point in the direction of the face 14 of the tool, while the north pole of magnet 25 is pointed away from the face 14. The central magnet may alternatively be reversed or omitted.
The cavity 28 is adapted for receiving an RF antenna 18 that is shown in FIG. 3. The antenna is positioned outside of the metal body 27 (FIG. 2) of the tool, and is thereby shielded from electromagnetic communication with regions of the borehole which lie behind the body 27, or regions of the other formations in directions intercepted by the body 27. Antenna 18 is thus responsive only to magnetic fields originating in front of the wall engaging face 14, e.g. fields originating in the formation 12 or in the mudcake or mud which contacts face 14 in the vicinity of the antenna 18. In a disclosed embodiment of the referenced patent, the body 27 is made of metal alloy sheathing, rigidly attached to interior metal bracing, which envelops most components of the tool other than the antenna 18, including the circuitry, the magnet array 17, and the hydraulics system of the arm 15. The patent points out that the body 27 can alternatively be constructed of other materials, so long as the overall structure is sufficiently strong and the magnetic field of the magnet array 17 can penetrate the body and enter the adjoining formation 12.
In the referenced patent, antenna 18 is used both as an RF transmitter to produce an oscillating magnetic field in formation 12 and as a receiving antenna to detect coherent magnetic signals emanating from precessing protons immediately after the oscillating field is terminated. The antenna serves effectively as a current loop which produces an oscillating field B.sub.1 within the volume of investigation that is perpendicular to B.sub.0.
Descriptions of the construction and functioning of the antenna are found in several U.S. Pat. Nos. (5,055,788 and 5,055,787) also assigned to the assignee of the present invention. Referring to FIG. 3, the antenna 18 comprises a highly conductive semi-cylindrical cavity or trough 29, end plates 30 and 31 and center conductor or probe 32 which extends from one end plate 30 to the other end plate 31, parallel to and centered in the semi-cylindrical trough 29. The trough 29, end plates 30 and 31 and antenna probe element 32 are indicated as preferably being of heavy gauge copper which has very low electrical resistance. Antenna probe element 32 is insulated from end plate 30 by a non-conducting bushing 33 and is connected to a conductor 34 on the other side of end plate 30. Probe 32 is attached at its other end to the other end plate 31 so that current passes freely between trough 29 and probe 32 via end plate 31. Conductor 34 is shown in FIG. 3 schematically as being connected to circuitry including an amplifier 35 and a detector 36. All connections in antenna 18 are stated to be brazed or silver soldered to ensure suitably low resistive loss. As described in the referenced patent, RF antenna 18 can be driven by amplifier 35 during specified periods of time (the signal being applied at conductor 34 with respect to the antenna body), during which it serves as an RF antenna transmitter. Alternatively, at other specified times, antenna 18 is electrically connected to detector 36, during which time it serves as an RF receiving antenna. In some modes of operation, antenna 18 may be called upon to alternatively function as transmitter or receiver in very rapid succession. The space between trough 29 and antenna element 32 is preferably filled with a nonconductive material 37 having high magnetic permeability. In order to increase the antenna sensitivity ferrite materials are preferably used. Several tuning capacitors 38 are connected between the base of antenna element 32 and the trough 29, with the capacitance thereof being chosen to produce an LC circuit, with the resonant frequency being the Larmor frequency .omega.L=.gamma.B.sub.0.
The radio frequency antenna of the described NMR logging device must operate with very high sensitivity to the received signal. The antenna is required to sense magnetic fields at a frequency on the order of 1 MHz and a magnitude of about 10.sup.-12 Tesla. The resultant voltage induced on the antenna is of the order of 10.sup.-8 volts. This tiny voltage must be sensed within about 10.sup.-4 seconds of the cessation of the driving signal on the antenna which will typically have an amplitude of hundreds of volts. When in use as a transmitter, the antenna should produce the largest possible field for a given amount of input power. All of these difficult requirements necessitate use of a high performance antenna. As previously described, a high magnetic permeability non-metallic insulating material such as ferrite is loaded in the antenna in order to increase antenna sensitivity. However, Applicant has found that the effectiveness of the ferrite is greatly reduced by the strong static magnetic field, which can saturate the ferrite and reduce its intrinsic permeability.
To address this issue, an antenna housing or shell of high magnetic permeability material is utilized. This material effectively provides a shunt path for static magnetic field in the region of the antenna that would otherwise have deleterious effect on the ferrite and on antenna operation. The high magnetic permeability material used to form the antenna shell can be for example a mild steel. The antenna shell provides foundation support for the conductor metal of the antenna body and serves as a low magnetic reluctance path that effectively shunts magnetic fields that could otherwise saturate the ferrite loaded antenna.
The relative dimensions of antenna 18 should be selected to maximize the antenna efficiency. The slot element radius R should be as large as practical, and the spacing R-r should be maximized subject to the condition that r must not be so small as to increase the antenna impedance excessively. It has been found that for a 12 inch trough antenna without ferrite filling, R=0.75 inch and r=0.2 inch produces optimum efficiency. A ferrite filled trough antenna having dimensions R=0.75 inch and r=0.3 inch has been found to be optimum. The length L of the antenna may be the same as the length of the magnet array 17, which is 12 inches in the preferred embodiment, but antenna 18 is preferably about the same length as the resonance region produced by the magnet array 17 in the formation, which is approximately 4 to 8 inches long.
Antenna 18 is used both as an RF transmitter to produce a polarizing magnetic field in formation 12 and as a receiving antenna to detect coherent magnetic signals emanating from precessing protons immediately after the polarizing field is terminated. Antenna 18 should be constructed of one or more current carrying loops which are highly efficient in generating magnetic fields in the formation. It is preferably made of a current loop which produces an oscillating field B.sub.1 within the volume of investigation which is perpendicular to B.sub.0. Other current loop orientations may be useful in other embodiments of the invention having a static field B.sub.0 differing from that of the preferred magnet array 17.
As previously stated, the antenna 18 is attached to body 27 and fitted within the slot 28. Its efficiency can be ideally maximized when the current density within the slot 28 is made uniform. In practice, optimum antenna efficiency is difficult to achieve because of various electromagnetic parasitic effects like the "skin effect", the mutual inductive effects between distinct current loops, and electrical effects within individual conductors.
RF antennas used in magnetic resonance applications mainly are comprised of ferrite and copper materials. Previous antenna designs involved bonding the various copper and ferrite components with epoxy. When this type of assembly was exposed to high temperature and pressure environments the assembly would delaminate. Void spaces would be found in the bond areas of the assembly. The same result occurred when the assembly was encapsulated with resin and cured under pressure. Absorption of oil was found to be a problem in the ferrites. Oil migrated under pressure into the ferrites and when the pressure was released the antenna assembly would delaminate. Typically other antennas have been fabricated by bonding with epoxy or rubber overmolding. The antennas would not withstand the environmental effects down hole because fluids or gas could migrate into the assembly during exposure to high temperature and pressure. Once gas or fluid pressure builds up in the void spaces it cannot be released quickly enough as the external pressure is relieved and the antenna component destroys itself. Still other antennas were assembled by gluing the parts together. Some parts have dissimilar thermal expansions, which cause the bonds to break under high temperatures. There is a need for an antenna assembly that is able to operate in high temperature and pressure environments without delaminating.